[BC] FM Exciters (Pic's)

[Harold Hallikainen]

Speaking of AFC... One of the more interesting FM exciter AFC circuits I
came across was in (I think) a Collins exciter. A typical AFC, of course,
used an FM detector to generate a correction voltage that was fed back to
the FM oscillator. The problem is that the FM detector was really no more
stable than the FM oscillator, so it could just as easily take you off
frequency as keep you on frequency. One fairly common way of dealing with
this was to hetrodyne (using a crystal oscillator) down to a 200kHz IF. A
1kHz drift in that FM detector (often a "pulse counting" demodulator) was
a much larger percentage error than it would be at carrier frequency, so
you were generally kept on frequency. This was used in the early AEL and
Sparta exciters I worked with. The Collins one, though, had a crystal
reference oscillator that was switched into the FM detector something like
once per second. The output of the FM detector used this crystal as its
reference, sort of clamping the output to zero when the crystal was
driving the detector, then detecting the offset when the FMO was driving
it. Clever (and complicated) circuit!

The first PLL I worked on was the Moseley PCL-303/C STL transmitter. I
haven't looked at exciter designs recently. Are they now either DSP or
numerically controlled oscillators?

Harold


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